Architecture
Object Manager is the centralized resource broker in the Windows NT line of Operating Systems, which keeps track of the resources allocated to processes. It is resource-agnostic and can manage any type of resource, including device and file handles. All resources are represented as objects, each belonging to a logical namespace for categorization and having a type that represents the type of the resource, which exposes the capabilities and functionalities via properties. An object is kept available until all processes are done with it; Object Manager maintains the record of which objects are currently in use via reference counting, as well as the ownership information. Any system call that changes the state of resource allocation to processes goes via the Object Manager.
Objects can either be Kernel objects or Executive objects. Kernel objects represents primitive resources such as physical devices, or services such as synchronization, which are required to implement any other type of OS service. Kernel objects are not exposed to user mode code, but are restricted to kernel code. Applications and services running outside the kernel use the Executive objects, which are exposed by the Windows Executive, along with its components such as the memory manager, scheduler and I/O subsystem. Executive objects encapsulate one or more kernel objects and exposes not only the kernel and kernel-mediated resources but also an expanded set of services than the kernel does. Applications themselves can wrap one or more Executive objects and surface objects that offer certain services. Executive objects are also used by the environment subsystems (such as the NT subsystem, the POSIX subsystem etc.) to implement the functionality of the respective environments.
Whenever an object is created or opened, a reference to the instance, called a handle, is created. Object Manager indexes the objects both by their names as well as the handles. But, referencing the objects by the handles is faster because the name translation can be skipped. Handles are associated with processes (by making an entry into the process' Handle table that lists the handles it owns), and can be transferred between processes as well. A process must own a handle to an object before using it. A process can own a maximum of 16,000,000 handles at one time. During creation, a process gains handles to a default set of objects. While there exists different types of handles - file handles, event handles and process handles - they only help in identifying the type of the target objects; not in distinguishing the operations that can be performed through them, thus providing consistency to how various object types are handled programmatically. Handle creation and resolution of objects from handles are solely mediated by Object Manager, so no resource usage goes unnoticed by it.
The types of Executive objects exposed by Windows NT are:
| Process | A collection of executable threads along with virtual addressing and control information. |
|---|---|
| Thread | An entity containing code in execution, inside a process. |
| Job | A collection of processes. |
| File | An open file or an I/O device. |
| File mapping object | A region of memory mapped to a file. |
| Access token | The access rights for an object. |
| Event | An object which encapsulates some information, to be used for notifying processes of something. |
| Semaphore/Mutex | Objects which serialize access to other resources. |
| Timer | An objects which notifies processes at fixed intervals. |
| Key | A registry key. |
| Desktop | A logical display surface to contain GUI elements. |
| Clipboard | A temporary repository for other objects. |
| WindowStation | An object containing a group of Desktop objects, one Clipboard and other user objects. |
| Symbolic link | A reference to other objects, via which the referred object can be used. |
Read more about this topic: Object Manager (Windows)
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